The goal of this proposal is to study the role of retinal axons in maintaining post-synaptic neurons in the visual system of the chick and hamster. First, quantitative studies will be continued at the light microscopic (LM) level on the consequences of eye-removal on the chick ventral lateral geniculate nucleus (GLv). I will measure the volume of the GLv, the cell cross-sectional area changes and cell loss following eye-removal in newly-hatched and embryonic chicks. Second, ablation of the iptic tectum and retina will be made simultaneously in embryos and newly-hatched chicks to learn the consequences of increasing the degree of deafferentation on the GLv. Third, electron microscopic (EMN) will be continued to learn the identity and frequency of occurrrnce of retinal synapses in the EMN as well as the identity of retinal and tectal synapses in the GLv. It has been suggested (Hamburger and Levi-Montalcini, 1950) that the number of synapses on a neuron determines the degree to which the neuron is maintained. This prediction will be tested by comparing the extent of transneuronal degeneration in both the GLv and EMN fro LM findings with the relative numbers of retinal and tectal synapses found in the nuclei in EM studies. Fourth, findings in the chick visual system will be compared with similar studies in the lateral geniculate nucleus (LGN) of the newborn and adult hamster. Fifth, attempts will be made to replicate findings that ipsilateral retinal axons proliferate in the ventral LGN follwoing neonatal eye removal. Sixth, I will examine quantitatively the extent to which the aberrant ipsilateral fibers are able to prevent transneuronal degeneration in the ventral LGN by measuring the volume of the ventral LGN, cell area and cell number and comparing these values wit ssmilar measurmentst in normal animals as well as in animals with no retinal input to the nucleus.